1. Field of the Invention
The present invention relates, in general, to mixers and, more particularly, to an improved and novel mixer for stirred autoclave reactors for producing polyethylene.
2. Discussion of the Prior Art
It is well recognized in the current technology that polyethylene may be produced within stirred autoclave reactors. Apparatus of this nature has been described in the patent literature over the past thirty years or more, for example, as early as Krase et al U.S. Pat. No. 2,396,791 granted to DuPont on Mar. 19, 1946; in Pugh et al U.S. Pat. No. 3,756,996 granted to National Distillers and Chemical Corporation on Sept. 4, 1973 and; more recently, in Platz et al U.S. patent application Ser. No. 714,451 filed Aug. 16, 1976 for Ethylene Polymerization Reactor, now U.S. Pat. No. 4,071,325 also assigned to National Distillers and Chemical Corporation.
In Pugh et al and Platz et al mixing is accomplished within the reaction vessel by a plurality of agitator blades attached to a rotatably mounted shaft which extends along the longitudinal axis of the reactor. The rotating shaft supports a number of different types of agitator blades mounted at various stages along the axis of the shaft, a first group of the blades being trapezoidal in configuration and extending radially outwardly from the shaft so as to sweep through an annulus representing a portion of the cross-sectional area of the reactor. A second group of agitator blades is provided with blades each having a pentagonal configuration, and extends radially outwardly from the shaft to a distance adjacent the inner cylindrical wall of the reactor. The respective sets of blades are angularly adjustable so as to move the reaction mixture generally in one direction or upstream through the annular portion of the reactor proximate the agitator shaft and in an opposite direction or downstream along a portion of the reaction chamber adjacent the inner cylindrical reactor wall, with the extent of these oppositely directed movement being adjustable to produce either localized radial mixing or non-localized axial mixing as required in the different zones of the reactor. Accordingly, the types of agitator blades employed, and the angular adjustments thereof, regulate the proportion of localized radial mixing relative to non-localized axial mixing.
In a modified, stirred autoclave reactor system of this type, the ethylene gas feed rates, the volume and dimensions of the reaction region, the pressure, the types of agitation employed in the reactor, and the catalysts and their mode of injection are selected so as to produce a predetermined temperature profile longitudinally along the reactor vessel. The temperature of the reactor materials increases along the length of the reactor from about 270.degree. to 575.degree. F., while the pressure is elevated, frequently in excess of 20,000 psi. When a particular temperatur profile is maintained, an ethylene polymerization product is formed having improved optical and processing characteristics which render it particularly suitable for use in film-forming and molding applications.
The reactor apparatus for polymerizing ethylene in the modified, stirred autoclave reactor system includes an elongated, either vertically or horizontally oriented vessel incorporating a generally cylindrical reaction chamber. The ratio of length to diameter (L/D) of the chamber generally is at least 15:1, and may be 40:1, or even higher. The ethylene is initially passed through a first reaction zone or stage within the reactor in which a first, low temperature catalyst is employed in the chemical reaction. The reaction mixture within the first zone or storage is agitated to produce both radial and axial or end-to-end mixing, thereby ensuring the presence of a substantially uniform reaction temperature throughout the zone. The reaction mixture then flows from the first stage into a second stage or zone where it is admixed with a plurality of catalysts including a second, intermediate temperature catalyst and a third, high temperature catalyst. The reaction mixture in the second stage is agitated to produce effective radial mixing thereof, with the degree of end-to-end mixing being decreased in that zone to thereby establish a temperature gradient extending longitudinally along the zone. The reaction mixture thereafter flows from the second stage into a third and last reaction stage or zone in which it is agitated to produce both radial and end-to-end mixing, with the degree of end-to-end mixing in the third zone being greater than in the contiguous portion of the second zone. The intensity of mixing of the reaction mixture in the third zone is increased to ensure that the catalyst is evenly distributed, thereby preventing the formation of hot spots caused by concentrations of catalyst and ensuring that the temperature of the reaction effluent egressing from the last stage or zone does not increase above a level of approximately 575.degree. F.
The reaction mixture which is removed from the lastmentioned zone is thereafter subjected to decompression and the polyethylene resin product is separated from the effluent stream.
Intense mixing is particularly required in the last or final zone of the reactor for the following reasons:
In a stirred autoclave reactor used to polymerize ethylene into polyethylene, the amount of polymer produced in the reactor varies in proportion to the temperature differential which is present between the ethylene feed gas and the reaction vessel discharge materials or effluent. When ethylene is polymerized in a reaction vessel at a rate of twenty-five thousand (25,000) pounds per hour, the production of polymer may be increased by fifty (50) pounds per hour for each five-degree Fahrenheit (5.degree. F.) increase in the temperature differential. Accordingly, it is desirable to maintain the difference in temperature between the feed gas and the reaction vessel effluent as large as possible as this maximizes the production of polyethylene. One approach to maintaining this temperature differential as large as possible is to operate the reactor with a low feed gas temperature. Unfortunately, the use of a very low temperature feed gas results in the production of resinous polymers which do not have optical and other characteristics as fine as polymers produced when utilizing feed gas at higher temperatures. Another approach to increasing polymer production is to elevate the temperature of the reaction vessel discharge materials or effluent as much as possible. However, the temperature of the product discharged from the reactor is limited, to some extent, by the effectiveness of the mixing of materials within the reactor, particularly in the final zone. Poor mixing within the reactor may result in the presence of unspent catalyst in the discharged effluent which is passed from the reactor through a discharge line to a high pressure separator. Unspent catalyst in the separator may cause a further chemical reaction therein which results in overheating of the discharged product. Overheating of the product usually results in its decomposition, obviously an undesirable end condition. Further, unspent catalyst in the reactor discharge line may cause the origination of a decomposition of the product therein, which then spreads to the bottom or discharge end of the reactor to cause a further decomposition of product. Effective intensive mixing within the reactor improves the temperature stability therein since it will evenly disperse the catalyst and reactor materials and also results in the most efficient use of the catalyst. Further, effective mixing within the reactor results in an even continuity of the polymerization reaction, in essence, product is more effectively homogenized as it is formed during the polymerization process in an exothermic reaction. This results in a reduction of hot spots within the reaction media so as to lessen the chance of the origination of a decomposition. Consequently, through the application of effective intensive mixing, the temperature of the effluent discharged from the reactor may be maintained at a higher level without undue fear that a decomposition may be triggered by a hot spot in the discharged materials caused by undispersed and unspent catalyst.